# Java - Regular Expression

## Introduction

Regular expressions are powerful and flexible tools for text processing. They allow you to define complex patterns of text which can be discovered in an input string, programmatically. Once these patterns are discovered, you can then react to them any way you wish. While the syntax of regular expressions can be difficult at first, they offer a lightweight and versatile language that can be used to solve all kinds of issues related to string processing, matching and selection, editing and verification in a general manner.

• A regular expression is a kind of pattern that can be applied to text (Strings, in Java)
• A regular expression either matches the text (or part of the text), or it fails to match
• If a regular expression matches a part of the text, then you can easily find out which part
• If a regular expression is complex, then you can easily find out which parts of the regular expression match which parts of the text
• With this information, you can readily extract parts of the text, or do substitutions in the text
• Regular expressions are an extremely useful tool for manipulating text
• Regular expressions are heavily used in the automatic generation of Web pages

For example, to find out if a number might or might not be preceded by a minus sign, you can use the following pattern: -?

A digit is defined in regular expressions, by \d. If you have some experience with other languages with standard expressions, you will instantly find a difference in how the backslashes are treated.

You use a '+' to indicate "one or more of the preceding expression." So to say "maybe a minus sign, followed by one or more digits," you 're writing: -?\\d+

Using the features built into the String class is the easiest way of using regular expressions. We will see for example whether a string matches the standard expression above:

public class MatchInteger {
public static void main(String[] args) {
System.out.println("-456".matches("-?\\d+"));
System.out.println("5678".matches("-?\\d+"));
System.out.println("+720".matches("-?\\d+"));
System.out.println("+720".matches("(-|\\+)?\\d+"));
}
}
/* Output:
true
true
false
true
*/


## split()

split() is a useful regular expression tool built into String class, meaning: "Split this string around regular expression matches."

import java.util.*;
public class Splitting {
public static String myString = "Then, Hong Kong is a 'safe harbor' for tech companies";
public static void split(String regex) {
System.out.println(Arrays.toString(myString.split(regex)));
}
public static void main(String[] args) {
split(" "); // Doesn’t have to contain regex chars
split("\\W+"); // Non-word characters
split("n\\W+"); // ‘n’ followed by non-word characters
}
}
/* Output:
[Then,, Hong, Kong, is, a, 'safe, harbor', for, tech, companies]
[Then, Hong, Kong, is, a, safe, harbor, for, tech, companies]
[The, Hong Kong is a 'safe harbor' for tech companies]


A common regular expression method implemented in String is replacement. You can replace only the first occurance, or all:

import java.util.*;
public class Splitting {
public static String myString = "Then, Hong Kong is a 'safe harbor' for 'tech companies'";
public static void main(String[] args) {
System.out.println(myString.replaceFirst("'\\w+", "'WHAT"));
System.out.println(myString.replaceAll("'\\w+", "'WHAT"));
}
}
/* Output:
Then, Hong Kong is a 'WHAT harbor' for 'tech companies'
Then, Hong Kong is a 'WHAT harbor' for 'WHAT companies'


## java.util.regex

Beginning with Java 1.4, Java has a regular expression package, java.util.regex

### Example

The regular expression [a-z]+ will match a sequence of one or more lowercase letters:

• [a-z] means any character from a through z, inclusive + means “one or more”

Suppose we apply this pattern to the String Now is the time

• There are three ways we can apply this pattern:
• To the entire string: it fails to match because the string contains characters other than lowercase letters
• To the beginning of the string: it fails to match because the string does not begin with a lowercase letter
• To search the string: it will succeed and match ow
• If applied repeatedly, it will find is, then the, then time, then fail
1. First, you must compile the pattern
import java.util.regex.*;
Pattern p = Pattern.compile("[a-z]+");

1. Next, you must create a matcher for a specific piece of text by sending a message to your pattern
Matcher m = p.matcher("Now is the time");

1. Points to notice:
• Pattern and Matcher are both in java.util.regex
• Neither Pattern nor Matcher has a public constructor; you create these by using methods in the Pattern class
• The matcher contains information about both the pattern to use and the text to which it will be applied
2. Now that we have a matcher m,
• m.matches() returns true if the pattern matches the entire text string, and false otherwise
• m.lookingAt() returns true if the pattern matches at the beginning of the text string, and false otherwise
• m.find() returns true if the pattern matches any part of the text string, and false otherwise
• If called again, m.find() will start searching from where the last match was found
• m.find() will return true for as many matches as there are in the string; after that, it will return false
• When m.find() returns false, matcher m will be reset to the beginning of the text string (and may be used again)

#### Finding what was matched

• After a successful match, m.start() will return the index of the first character matched
• After a successful match, m.end() will return the index of the last character matched, plus one
• If no match was attempted, or if the match was unsuccessful, m.start() and m.end() will throw an IllegalStateException
• This is a RuntimeException, so you don’t have to catch it
• It may seem strange that m.end() returns the index of the last character matched plus one, but this is just what most String methods require
• For example, Now is the time".substring(m.start(), m.end()) will return exactly the matched substring
import java.util.regex.*;
public class RegexTest {
public static void main(String args[]) {
String pattern = "[a-z]+";
String text = "Now is the time";
Pattern p = Pattern.compile(pattern);
Matcher m = p.matcher(text);
while (m.find()) {
System.out.print(text.substring(m.start(), m.end()) + "*");
}
}
}

Output: ow*is*the*time*


In the above example, we saw the basic methods of a matcher m. Here some additional methods:

• m.replaceFirst(replacement) returns a new String where the first substring matched by the pattern has been replaced by replacement
• m.replaceAll(replacement) returns a new String where every substring matched by the pattern has been replaced by replacement
• m.find(startIndex) looks for the next pattern match, starting at the specified index
• m.reset() resets this matcher
• m.reset(newText) resets this matcher and gives it new text to examine (which may be a String, StringBuffer, or CharBuffer)

## Some simple patterns

With a subset of the possible constructs you can start learning regular expressions. A full list of constructs can be found in the JDK documentation for the java.util.regex package pattern class for constructing regular expressions.

PatternDescription
BThe specific character B
abcexactly this sequence of three letters
[abc]Any of the characters a, b, or c (same as a|b|c)
[^abc]Any character except a, b, and c (negation)
[a-zA-Z]Any character a through z or A through Z (range)
[abc[hij]]Any of a,b,c,h,I,j (same as a|b|c|h|i|j) (union)

## Sequences and alternatives

If one pattern is followed by another, the two patterns must match consecutively.

• For example, [A-Za-z]+[0-9] will match one or more letters immediately followed by one digit

The vertical bar, |, is used to separate alternatives

• For example, the pattern abc|xyz will match either abc or xyz

## Some predefined character classes

PatternDescription
\tTab
\nNewline
\rCarriage return
\fForm feed
\eEscape
.Any character
\sA whitespace character (space, tab, newline, form feed, carriage return)
\SA non-whitespace character ([^\s])
\dA numeric digit [0-9]
\DA non-digit [^o-9]
\wA word character [a-zA-Z_0-9]
\WA non-word character [^\w]

## Boundary matchers

These patterns match the string if at the specified position:

PatternDescription
^the beginning of a line
\$the end of a line
\ba word boundary
\Bnot a word boundary
\Athe beginning of the input (can be multiple lines)
\Zthe end of the input except for the final terminator, if any
\zthe end of the input

## Greedy quantifier

A greedy quantifier will match as much as it can, and back off if it needs to. Assume X represents some pattern:

PatternDescription
X?optional, X occurs once or not at all
X*X occurs zero or more times
X+X occurs one or more times
X{n}X occurs exactly n times
X{n,}X occurs n or more times
X{n,m}X occurs at least n but not more than m times

## Reluctant quantifier

A reluctant quantifier will match as little as possible, then take more if it needs to. You make a quantifier reluctant by appending a ?:

• X??
• X*?
• X+?
• X{n}?
• X{n,}?
• X{n,m}?

## Possessive quantifier

A possessive quantifier will match as much as it can, and never let go. You make a quantifier possessive by appending a +:

• X?+
• X*+
• X++
• X{n}+
• X{n,}+
• X{n,m}+

## Quantifier examples

Suppose your text is aardvark

• Using the pattern a*ardvark (a* is greedy):
• The a* will first match aa, but then ardvark won’t match
• The a* then “backs off” and matches only a single a, allowing the rest of the pattern (ardvark) to succeed
• Using the pattern a*?ardvark (a*? is reluctant):
• The a*? will first match zero characters (the null string), but then ardvark won’t match
• The a*? then extends and matches the first a, allowing the rest of the pattern (ardvark) to succeed
• Using the pattern a*+ardvark (a*+ is possessive):
• The a*+ will match the aa, and will not back off, so ardvark never matches and the pattern match fails

## Capturing groups

In regular expressions, parentheses are used for grouping, but they also capture (keep for later use) anything matched by that part of the pattern

• Example: ([a-zA-Z]*)([0-9]*) matches any number of letters followed by any number of digits
• If the match succeeds, \1 holds the matched letters and \2 holds the matched digits
• In addition, \0 holds everything matched by the entire pattern

Capturing groups are numbered by counting their opening parentheses from left to right:

( ( A ) ( B ( C ) ) )
1 2     3   4
\0 = \1 = ((A)(B(C))),   \2 = (A),   \3 = (B(C)),   \4 = (C)


Example: ([a-zA-Z])\1 will match a double letter, such as letter

### Capturing groups in Java

If m is a matcher that has just performed a successful match, then

• m.group(n) returns the String matched by capturing group n
• This could be an empty string
• This will be null if the pattern as a whole matched but this particular group didn’t match anything
• m.group() returns the String matched by the entire pattern (same as m.group(0))
• This could be an empty string

If m didn’t match (or wasn’t tried), then these methods will throw an IllegalStateException

### Example of capturing groups

Suppose word holds a word in English. Also suppose we want to move all the consonants at the beginning of word (if any) to the end of the word (so string becomes ingstr)

Pattern p = Pattern.compile("([^aeiou]*)(.*)");
Matcher m = p.matcher(word);
if (m.matches()) {
System.out.println(m.group(2) + m.group(1));
}


Note the use of (.*) to indicate all the rest of the characters

## Double backslashes

• Backslashes have a special meaning in regular expressions; for example, \b means a word boundary
• Backslashes have a special meaning in Java; for example, \b means the backspace character
• Java syntax rules apply first!
• If you write "\b[a-z]+\b" you get a string with backspace characters in it--this is not what you want!
• Remember, you can quote a backslash with another backslash, so "\\b[a-z]+\\b" gives the correct string
• Note: if you read in a String from somewhere, this does not apply--you get whatever characters are actually there

## Escaping metacharacters

• A lot of special characters--parentheses, brackets, braces, stars, plus signs, etc.--are used in defining regular expressions; these are called metacharacters
• Suppose you want to search for the character sequence a* (an a followed by a star)
• "a*"; doesn’t work; that means “zero or more as”
• "a\*"; doesn’t work; since a star doesn’t need to be escaped (in Java String constants), Java just ignores the \
• "a\\*" does work; it’s the three-character string a, , *
• Just to make things even more difficult, it’s illegal to escape a non-metacharacter in a regular expression

## Spaces

• There is only one thing to be said about spaces (blanks) in regular expressions, but it’s important:

Spaces are significant!

• A space stands for a space--when you put a space in a pattern, that means to match a space in the text string
• It’s a really bad idea to put spaces in a regular expression just to make it look better

## String class

All of the following are public methods available in the string class for Regex:

 public boolean matches(String regex)
public String replaceFirst(String regex, String replacement)
public String replaceAll(String regex, String replacement)
public String[ ] split(String regex)
public String[ ] split(String regex, int limit)

• If the limit n is greater than zero then the pattern will be applied at most n - 1 times, the array's length will be no greater than n, and the array's last entry will contain all input beyond the last matched delimiter.
• If n is non-positive then the pattern will be applied as many times as possible

## Exercise

1. Exercise:
2. Exercise:
• validate IP address using regular expression.
3. Exercise:
• validate Email address using regular expression.